Cutting work on steel products, castings, and the like has conventionally been carried out using a cutting tool formed of cemented carbide. Such cutting tool often suffers from a problem of wear or fracturing damage of the cutting edge during cutting work due to exposure of the cutting edge to a tough environment, such as high temperature and high pressure. Thus, the cutting tool has a problem in cutting performance.
Therefore, a coating for coating a surface of a base member, such as one formed of cemented carbide, has been under development for the purpose of improving cutting performance of cutting tools. Among others, a coating formed of a compound of titanium (Ti), aluminum (Al), and nitrogen (N) (hereinafter also referred to as “TiAlN”) can have high hardness, and can improve oxidation resistance by increasing the content ratio of Al. Since performance of a cutting tool can be improved by coating the cutting tool with such a coating, further development of such coating is demanded.
For example, PTD 1 discloses a hard coating that includes at least one Ti1-xAlxN hard coating produced by means of chemical vapor deposition (CVD) without plasma excitation. The Ti1-xAlxN hard coating is present as a monophase layer of a cubic NaCl structure having a stoichiometric coefficient of x>0.75 up to x=0.93 and a lattice constant afcc of from 0.412 nm to 0.405 nm; or the Ti1-xAlxN hard coating has a primary phase formed of Ti1-xAlxN having a cubic NaCl structure having a stoichiometric coefficient of x>0.75 up to x=0.93 and a lattice constant afcc of from 0.412 nm to 0.405 nm. In addition, the Ti1-xAlxN hard coating is a multiphase layer including, as an additional phase, Ti1-xAlxN in a wurtzite structure and/or TiNx in a NaCl structure, and the chlorine content rate of Ti1-xAlxN hard coating is in a range of from 0.05 to 0.9 atomic %. NPD 1 also discloses a similar technique.
NPD 2 discloses a Ti0.05Al0.95N film having a thickness of 5 μm, deposited on a substrate such as one formed of WC—Co by a CVD method using AlCl3, TiCl4, N2, and NH3 as the reactant gases, and H2 as the carrier gas, under conditions of a pressure of 3 kPa and a temperature of 800° C. The Ti0.05Al0.95N film of NPD 2 has a nanolaminate structure having a self-organized cubic TiN (c-TiN) layer and a wurtzite AlN (w-AlN) layer being alternately stacked, and a separation region formed of w-AlN and cubic AlN (c-AlN). In this nanolaminate structure, a (110) plane of c-TiN and a (100) plane of w-AlN are parallel to each other. The ratios of w-AlN, c-AlN (c-Al(Ti)N), and c-TiN that form the Ti0.05Al0.95N film of NPD 2 are 53%, 26%, and 21%, respectively. NPD 2 also discloses that the hardness of the Ti0.05Al0.95N film is about 28 GPa, and that the compressive residual stress of c-Al(Ti)N is −1.2±0.1 GPa.
NPD 3 discloses evaluation of oxidation resistance of a Ti0.05Al0.95N film having a nanolaminate structure having a self-organized c-TiN layer and a w-AlN layer being alternately stacked. According to the description of NPD 3, when the Ti0.05Al0.95N film was oxidized at temperatures from 700° C. to 1200° C. in air for one hour, the Ti0.05Al0.95N film had good oxidation resistance up to 1050° C., whereas local degradation of the surface occurred after the temperature exceeded 1100° C. NPD 3 also discloses that the hardness of about 29 GPa and the compressive residual stress of 2 GPa of the Ti0.05Al0.95N film are maintained up to a temperature of 1050° C.
PTD 2 discloses a method for producing, by a CVD method, a hard coating having a structure in which a TiN layer of 2 nm thick having a face-centered cubic lattice (fcc) structure and an AlN layer of 6 nm thick having an fcc structure are alternately stacked, by introducing AlCl3 gas, TiCl4 gas, NH3 gas, H2 gas, and N2 gas into a reaction vessel at a pressure of 1.3 kPa and a temperature of 800° C., and then cooling the reaction vessel at a cooling rate of 10° C./min until the temperature of the base member reaches 200° C. (see paragraphs 0062 and 0063 of PTD 2).